New Equipment Deployed to Map Sea-Floor Geology in Southern Maine

Above: The research vessel Rafael departs Wells harbor early on a foggy Maine morning. Rough seas caused flexing of the bow-mounted sonar head, which was reconfigured the next day and deployed farther aft on a side boom. During surveys, the sonar head is lowered into the water, where it rides about 3 ft under the surface. Photograph by Erika Hammar-Klose. [larger version]

Above: Shaded-relief bathymetric map of Wells Embayment. Inset shows the location of the study area in the western Gulf of Maine. [larger version]

The arcuate embayment at Wells, Maine, is bounded by rocky headlands and contains sandy barrier beaches that support an important tourism industry in the area. The barriers also protect extensive salt marshes that compose part of the Rachel Carson National Wildlife Reserve and the Wells National Estuarine Research Reserve. In recent years, loss of sand from these beaches has accelerated, with serious economic and social consequences.

In cooperation with Joe Kelley from the University of Maine, scientists from the U.S. Geological Survey (USGS)'s Woods Hole Science Center conducted a research cruise on the 26-ft research vessel Rafael to map the geology of the inner continental shelf in early June. The survey provided an opportunity to test new transducers that were recently acquired for the interferometric sonar (Submetrix SwathPlus). Backscatter data were also collected with a sidescan sonar (Klein 3000), and subbottom profiles with a chirp seismic-reflection system (Edgetech 424). The USGS team was led by Walter Barnhardt and benefited from the superb seamanship of Barry Irwin, the technical expertise of Chuck Worley and Bill Danforth, and the data-processing wizardry of Erika Hammar-Klose. Special acknowledgment also goes to Tommy O'Brien, who broke landspeed records to deliver spare parts.

The lower-frequency (117 kHz) transducers provided high-resolution bathymetric data in the relatively deep water of Wells Embayment, where depths range from 20 to 60 m. As expected with new gear, problems arose to challenge our technical staff, including interference from the Klein sidescan sonar (100/500 kHz) and a bow-mounted sonar head that threatened to wreck the Rafael. Despite these and weather-related problems, we fully imaged 102 km2 of the inner continental shelf and collected about 400 km of seismic-reflection profiles.

Preliminary data from the cruise reveal a complex sea floor with high-relief bathymetry that is characteristic of formerly glaciated continental shelves. The extremely heterogeneous sea-floor environments were influenced by a complex history of deglaciation, relative sea-level change, and sediment reworking since the end of the latest Ice Age, about 14,000 years ago. Progressive northward retreat of the Laurentide Ice Sheet is recorded by a series of lobate, seaward-convex ridges of till (moraines) in nearshore areas. Erosion of these moraines, which occurred during multiple cycles of sea-level change, probably has been the main contributor of sand to the local sediment budget.

These data will be compiled with existing geophysical, sample, and core data from the University of Maine to produce interpretive geologic maps of the inner continental shelf. Detailed bathymetric data and geologic maps will provide a physical framework for research and management of the Maine coastal zone.